The hot forming process includes two ways: large diameter seamless hot rolling and hot extrusion. The former is mainly used for the production of composite pipe joints, while the latter is suitable for the production of seamless composite pipe.


Rolling is a traditional method to produce large diameter seamless steel tubes. Hot-rolled composites are basically lower than pressure welding. If the deformation is large enough, the pressure exerted by the metal roll will damage the oxide film surface and contact with the surface atoms to weld the two surfaces together. The advantages and disadvantages of rolling are as follows:
Advantages: High productivity, good quality, low cost, can save a lot of metal material losses, so the application of composite material production technology is very extensive. Roller bonded composite plate, accounting for 90% of the total composite plate, is often used in the processing of pipe wall thickness less than 32 mm.
(2) Disadvantage: One-time investment, there are many material combinations can not be achieved by rolling complex. At present, the most widely used process is rolling carbon steel, stainless steel composite pipe joint manufacturing.


Hot extrusion of bimetallic tubes is usually called compound extrusion (co-extrusion). Composite extrusion is the best method to produce seamless composite tubes of stainless steel and high nickel alloy at present. There are three methods to produce composite tubes before extrusion: forging billet perforation and hot extrusion enlargement; direct centrifugal rotary casting; and corrosion-resistant powder particles. Both internal and external metal materials use powder, which is called "NUVAL" technology. New alloys can be developed, but the cost of powder preparation is too high. The disadvantages of co-extrusion are:
Advantages: Metallurgical bonding is adopted at the interface; the force involved in the extrusion process is complete stress, which is especially suitable for hot working and low plasticity of high alloy metals.
(2) Disadvantage: Because the combination depends on the interfacial diffusion of extrusion process elements in a very short period of time, usually due to the existence of oxide film, it is limited to composite extrusion of carbon steel, stainless steel and alloy between high nickel. It should be noted that the deformation resistance of hot extrusion is small, allowing each large deformation, resulting in higher surface roughness, so there is a manufacturing method of rolling to hot extrusion (or stretching) composite pipe.